1. Field of the Invention
The present invention relates to an electromagnetic wave shield to be used in an apparatus applying a high frequency electromagnetic field.
2. Description of the Related Art
Equipment applying a high frequency electromagnetic field has been utilized in a variety of fields, for example, a cooking device using induction heating, a fixing component for a laser printer, a discharge lamp using a plasma induced by an induction field or a communicating device for transferring an energy in non-contact which uses an induction field. In an information terminal such as a computer including a cell phone, moreover, the influence of an electromagnetic wave radiated secondarily from the electronic equipment cannot be disregarded due to a rapid increase in a radio information transmission or a further increase in a driving frequency, and the regulation of an unnecessary radiation, that is, the regulation of the discharge of an unnecessary electromagnetic wave has been stricter.
In the equipment applying a high frequency electromagnetic field, an RFID system for carrying out a communication with a non-contact IC, card including an antenna coil in non-contact by utilizing an electromagnetic induction has got into the limelight. In this system, a high frequency magnetic flux is utilized for the communication. When a communicating antenna is to be driven, a high frequency electric field is also discharged in addition to a high frequency magnetic flux. The intensity of the high frequency electric field is regulated by the Radio Law. In order to satisfy the Radio Law, measures have been taken, for example, the output of the antenna has been decreased. In this case, however, there is a problem in that a communication distance is shortened. For other measures, a shield plate is provided around the antenna.
FIG. 13 is a schematic perspective view showing a conventional electromagnetic wave shield. According to (JP-A-2001-326526), for example, there has been disclosed a shield antenna coil 23 including an electric field shield pattern 21 and a feeding pattern coil 22 as shown in FIG. 13. The electric field shield pattern 21 has such a width as to cover the feeding pattern coil 22, and is set to be an open loop in order to prevent the generation of an eddy current to block the radiation of a magnetic flux component and is provided to cover the feeding pattern coil 22, and serves as a ground as shown in the drawing. By such a structure, it is possible to decrease such a magnetic field component as to obstruct the communication of other radio devices while maintaining a necessary magnetic flux component for a communication.
In these conventional structures, however, there is a problem in that the electric field can be decreased and the attenuation of a vicinal magnetic flux required for the communication is also increased, resulting in an extreme reduction in a communication distance.